Manganese Bismuth (MnBi) is an attractive alternative to permanent magnets containing rare earth elements such as NdFeB—Dy and SmCo used in medium-temperature (423 K to 473 K) applications. MnBi has unique temperature properties. For example, MnBi has a coercivity (Hc) value that increases with increasing temperature, reaching a maximum of 2.6 T at 523 K (250° C.). This large coercivity is attributed to MnBi's large magnetocrystalline anisotropy (1.6×106 J/m3). MnBi has a relatively low magnetization value. At room temperature, its saturation magnetization is about 75 emu/g or 8.4 kG in a 5 T field. The corresponding maximum theoretical energy product (BH)max is about 17.6 MGOe. The roadmap for developing a MnBi-based magnet starts with preparing a high purity MnBi compound in a large quantity. However, synthesizing MnBi is a challenge. Melting temperatures of Mn and Bi are 1519 K (1246° C.) and 544 K (271° C.), respectively. The Mn—Bi phase diagram (ASM Alloy Phase Diagram Database, ASM International, Materials Park, Ohio, USA) shows that undesired peritectic reactions occur over a wide range of temperatures and compositions. Processes are further complicated by a eutectic reaction that occurs between liquid bismuth (Bi) metal and solid MnBi at a temperature of 535 K (262° C.), which limits the maximum temperature to which composite materials can be exposed. While this eutectic temperature is about 112 K higher than the desired operating temperature of 423 K (150° C.), it is low for fabrication methods that include sintering and hot pressing for typical bulk magnets.
Several parameters are used to characterize a magnetic material: remnant magnetization (Br), coercivity force (Hc), and maximum energy product ((BH)max). The (Br) value is a measure of magnet strength in the absence of an external magnetic field. The coercivity force or value (Hc) is a measure of a magnetic material's ability to remain magnetized in an external field. (BH)max represents the maximum product between an induced magnetization value and a corresponding applied field. However, a high (Br) value or a high (Hc) value does not mean a high (BH)max value, as many magnetic materials retain either a high (Br) value or a high (Hc) value, but not both. TABLE 1 lists properties of several important magnetic materials, including MnBi.
TABLE 1lists magnetic properties of common magnetic materials.MagnetizationCoercivityEnergy Product(Br) kG(Hc) kOe(BH)max MGOeFe14Nd2B121240AlNiCo-910.51.68.5MnBi4.8157.7Fe21.50.0010.02FeCo24.50.0020.05
“Hard” magnetic materials do not magnetize or de-magnetize easily. “Soft” magnetic materials magnetize and de-magnetize easily. A magnetic material is considered “hard” if its coercivity (Hc) is greater than 1000 Oe, and “soft” if the (Hc) value is less than 100 Oe. Generally, “hard” permanent magnets have a coercivity value greater than 3000 Oe, and, in some case, a coercivity value over 10,000 Oe. “Soft” magnetic materials typically exhibit a coercivity (Hc) less than 10 Oe, and, in some cases, a coercivity (Hc) of 0.1 Oe.
Several conventional approaches are used to prepare single-phase MnBi materials, including arc-melting, sintering, and melt-spin/rapid solidification. Of these approaches, only melt-spin/rapid solidification has been able to consistently produce low-temperature phase (LTP) MnBi, also referred herein as α-MnBi, at a purity over 90%. In this approach, rapid cooling freezes MnBi in an amorphous phase. Subsequent heat treatment allows the amorphous phase to crystallize yielding fine grains of MnBi at a 1:1 stoichiometric ratio. The advantage of the melt-spin/rapid solidification approach is the high purity and high quality of the obtained material. However, the approach has a significant disadvantage in that productivity (i.e., productive yields) is low and the cost is high. For example, the melt-spin process involves injecting molten metal onto a rapidly rotating wheel, which throws solidified metal into a chamber. The injection and the throwing actions provide a continuous ribbon of the high-purity material. However, due to the necessity of maintaining high temperatures and the formation of the thin ribbon product obtained, the productivity of the approach is limited.
In another powder metallurgy approach, powders of Mn and Bi are mixed and then sintered to produce LTP phase MnBi. However, this approach provides a yield of α-MnBi that is less than 50%. And, the α-MnBi phase material is not easily separated from unreacted manganese (Mn) and bismuth (Bi) metal phases in the composite material.
In yet another approach, LTP phase MnBi is produced via conventional casting followed by heat treatment. In this approach, after arc melting, the obtained ingot is annealed at 300° C. for 24 hours. The powder obtained exhibits a saturation magnetization of 60 emu/g in an applied field of 30 kOe at room temperature, which is equivalent to a purity of MnBi of 74%, assuming the magnetization of 100% pure LTP MnBi is 81 emu/g in an applied field of 30 KOe. Results show that simple annealing cannot produce LTP phase MnBi at a purity greater than 90%.
Accordingly, new processes are needed that produce mass (kg) quantities of high-purity (>90%) MnBi magnetic materials with suitable properties for energy production applications. The present invention addresses these needs.